This application claims the benefit of Korean Application No. P2000-86333 filed on Dec. 29, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a method for crystallizing amorphous silicon, and more particularly, to method for crystallizing amorphous silicon and fabricating a thin film transistor using the crystallized silicon.
2. Discussion of the Related Art
Due to a rapid development in information technologies, display devices are developed in accordance with the pace of the technology development. Display devices process and display a great deal of information. A cathode ray tube (CRT) has served as a mainstream of the display devices. To meet the needs of the current technology break-through, a flat panel display device having small size, light weight, and low power consumption is a more important area in research.
In general, liquid crystal display (LCD) devices make use of optical anisotropy and polarization characteristics of liquid crystal molecules to control an arrangement in orientation. The arrangement direction of the liquid crystal molecules can be controlled by applying an electric field. Accordingly, when the electric field is applied to the liquid crystal molecules, the arrangement of the liquid crystal molecules changes. Since refraction of incident light is determined by the arrangement of the liquid crystal molecules, display of image data can be controlled by changing the electric field applied to the liquid crystal molecules.
As an active layer of a thin film transistor (TFT) in an array substrate for use in liquid crystal display (LCD) devices, amorphous silicon (a-Si) is widely used. This is because amorphous silicon can be formed on the low cost glass substrate at a low temperature in fabricating a large LCD panel. However, a driving circuit is required to drive the thin film transistors including amorphous silicon.
As well known, the liquid crystal display device includes an array substrate, and the array substrate is electrically connected to large scale integrated circuit (LSIC) fabricated by the single crystal silicon, using a tape automated bonding (TAB) method. The driving circuit, however, is very expensive, and thus the liquid crystal display including the large scale integration costs also high.
Accordingly, the thin film transistor (TFT) formed of polycrystalline silicon (poly-Si) for fabricating a liquid crystal display device has been researched and developed. In the liquid crystal display device employing poly-Si in a thin film transistor, the thin film transistor and the driving circuit can be formed on the same substrate. Thus, it is not necessary for the driving circuit to be connected to the thin film transistor. Further, it is easy to obtain a fast response time in display when using the polycrystalline silicon as an element of the TFT rather than the amorphous silicon as an element of the TFT. Namely, a field effect mobility in poly-Si is 100 to 200 times faster than that in a-Si. Additionally, the poly-Si has a good stability against light and temperature variations.
In the view of the foregoing, various methods for forming the poly-Si are well known. Of the different types of methods for forming poly-Si, a method for crystallizing a-Si after depositing a-Si using plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD) is widely known and employed in the crystallization of a-Si. Furthermore, as a method for forming poly-Si from a-Si, for example, a solid phase crystallization (SPC), an excimer laser crystallization (ELC) or a metal induced crystallization (MIC) has been employed.
In the process of the solid phase crystallization (SPC), amorphous silicon is exposed to heat approaching about 600 degrees celsius for at least several hours. Namely, the solid phase crystallization (SPC) is to change amorphous silicon into polycrystalline silicon by a heat-treatment at a high temperature for a long time in a furnace. It requires forming a buffer layer on the quartz substrate having a thermal endurance over temperatures of 600 degrees celsius (xc2x0 C.) in order to prevent the quartz substrate from diffusing out impurities. The amorphous silicon layer is deposited on the buffer layer and introduced by the heat-treatment.
The solid phase crystallization (SPC) method, however, results in an irregular grain growth and irregular grain boundaries so that the gate insulating layer on the polycrystalline silicon layer grows erratically, thereby lowering breakdown voltage of the device. In addition, since the grain sizes of the polycrystalline silicon are excessively non-uniform, electrical characteristics, such as current and a threshold voltage, are not good. Further, a costly quartz substrate should be employed.
The excimer laser crystallization (ELC) process has also been used with some advantages in annealing amorphous silicon. Laser allows areas of the amorphous film to be exposed to very high temperatures for very short periods of time. Theoretically, this offers a possibility of annealing the amorphous silicon at an optimum temperature (less than 400 degrees celsius) without degrading the transparent substrate upon which it is mounted. However, use of this method has been limited by the lack of control over some of the process steps. Typically, an aperture size of the laser is relatively small. The aperture size, power of the laser, and a thickness of the film may require multiple laser passes, or shots, to complete an annealing process. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the annealing process. Further, the substrates must be annealed serially, instead of in a furnace. TFTs made by this method are significantly more expensive than those made by direct deposition or SPC.
A metal induced crystallization is another example of a method for crystallizing amorphous silicon at a low temperature. In the metal induced crystallization, metal, such as nickel (Ni), is disposed on the amorphous silicon layer, and then lowers the crystallization temperature of the amorphous silicon.
For a better understanding of the metal induced crystallization (MIC) method, detailed descriptions are made with reference to the accompanying drawings as follows.
FIGS. 1A to 1C are perspective views showing crystallization steps of an amorphous silicon layer using a field effect metal induced crystallization (FEMIC) method according to a related art.
Referring to FIG. 1A, a buffer layer 2 is formed on a substrate 1, and then an amorphous silicon layer 4 is formed on the buffer layer 2. The buffer layer 2 is generally formed of silicon oxide (SiO2) and protect the amorphous silicon layer 4 from the alkali substances. In FIG. 1A, metal clusters 8, such as nickel (Ni), paladium (Pd), iron (Fe), and cobalt (Co), is deposited on the entire surface of the amorphous silicon layer 4 by evaporation, sputtering, CVD or using a metal solution. This metal is called a metal catalyst and heated by a high density voltage.
FIG. 1B shows a step of crystallizing the amorphous silicon layer 4. As shown, a pair of electrodes 10 are disposed at both sides of the amorphous silicon layer 4 including the metal clusters 8 thereon. Thereafter, a direct current having a high voltage is supplied to the electrodes 10. While applying the direct current having high voltage to the electrodes 10, the substrate 1 is then subjected to a thermal treatment with a temperature of approximately 500 degrees celsius (xc2x0 C.). As a result, a polycrystalline silicon layer 5 of FIG. 1C is formed over the substrate 1.
During the above-mentioned field enhanced metal induced crystallization (FEMIC) method, the metal catalyst such as nickel (Ni) on the amorphous silicon layer 4 begins to react with the amorphous silicon at a temperature of about 200 degrees celsius (xc2x0 C.), and then is converted into silicide. At the beginning of the reaction, nickel can be easily silicified by heating at about 200 degrees celsius (xc2x0 C.), thereby forming Ni2Si. As the temperature increases, nickel (Ni) becomes nickel monosilicide (NiSi), and finally formed as nickel disilicide (NiSi2). The final phase, nickel disilicide (NiSi2), is the most stable state. As a result, the amorphous silicon is crystallized into polycrystalline silicon (poly-Si) at a temperature of about 500 degrees celsius (xc2x0 C.) because nickel disilicide (NiSi2) acts as seeds in crystallization. When crystallizing the amorphous silicon layer 4, if the voltage is applied to the electrodes 10, the crystallization of amorphous silicon is enhanced because a diffusion rate and a mobility of nickel disilicide (NiSi2) are promoted. Thus, the poly-Si layer can be formed at a low temperature for a relatively short time.
However, there are some problems in the conventional field enhanced metal induced crystallization (FEMIC) method. An infinitesimal quantity of metal catalyst is required to prevent the metal catalyst from remaining in the polycrystalline silicon layer. The residual metal catalyst causes a deterioration of the polycrystalline silicon layer. Further, the metal and amorphous silicon are oxidized during the crystallization. Namely, the metal is combined with oxygen and the surface of the amorphous silicon layer is oxidized, thereby decreasing a crystallization rate of the amorphous silicon.
Furthermore, during the crystallization processes, heat loss occurs in the entire surface of the amorphous silicon layer. Therefore, the crystallized polysilicon at a temperature of about 500 degrees celsius (xc2x0 C.) has irregular or non-uniform grains after crystallization. That is due to an irregular temperature distribution in the amorphous silicon layer.
Accordingly, the present invention is directed to a method for crystallizing an amorphous silicon and fabricating a thin film transistor using crystallized silicon that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a method for crystallizing an amorphous silicon layer by enhancing a crystallization rate.
Another object of the present invention is to provide a method for crystallizing an amorphous silicon layer at a relatively low temperature.
A further object of the present invention is to provide a method for crystallizing an amorphous silicon layer having uniform grains.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for crystallizing amorphous silicon includes forming an amorphous silicon layer over a substrate, forming a plurality of metal clusters the amorphous silicon layer, and simultaneously applying a thermal treatment, an electric field, and a magnetic field to crystallize the amorphous silicon layer.
The method for crystallizing amorphous silicon further includes forming a buffer layer between the substrate and the amorphous silicon layer. Here, the buffer layer is formed of silicon oxide (SiO2)
The above-mentioned the amorphous silicon layer is crystallized at a temperature of about 450 degrees celsius (xc2x0 C.)
In the method for crystallizing amorphous silicon, the magnetic field may be applied to a direction facilitating a movement of atoms of the metal clusters. The metal clusters are metal selected from a group consisting of nickel (Ni), paladium (Pd), iron (Fe), and cobalt (Co).
In another aspect of the present invention, a method for forming a thin film transistor includes forming a buffer layer on a substrate, forming an amorphous silicon layer on the buffer layer, forming a plurality of metal clusters on the amorphous silicon film, simultaneously applying a thermal treatment, an electric field, and a magnetic field to crystallize the amorphous silicon layer into a polycrystalline silicon layer, patterning the polycrystalline silicon layer to form an island shape, forming a gate insulating layer on the island-shaped polycrystalline silicon, layer, forming a gate electrode on the gate insulation layer, forming an active region, a source region, and a drain region in the island-shaped polycrystalline silicon layer, forming an interlayer insulating layer on the gate electrode and the island-shaped polycrystalline silicon layer including the buffer layer, patterning the interlayer insulating layer to form a source contact hole at the source region and a drain contact hole at the drain region, and simultaneously forming a source electrode contacting the source region through the source contact hole and a drain electrode contacting the drain region through the drain contact hole.
The above-mentioned dopant includes a group III element, such as boron (B), or a group V element, such as phosphorous (P).
In the method for forming the thin film transistor, the thermal treatment is performed at a temperature of about 450 degrees celsius (xc2x0 C.). Further, the magnetic field is applied to a direction facilitating a movement of atoms of the metal clusters.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.